This invention relates to improvements on a puffer type gas circuit breaker using a puffer piston cylinder assembly.
To date, a puffer type gas circuit breaker has been applied to shut off a large current in a short time and reliably. With the prior art circuit breaker of the above-mentioned type, a movable arcing contact 1 which is formed of a plurality of arcing contact fingers 2 arranged in a ring form and surrounded by a nozzle 3 prepared from electrically insulating material is fixed to the outer end of a movable puffer cylinder 4, into which a stationary puffer piston 5 is inserted. A stationary arcing contact 6 is provided to concentrically face the movable arcing contact 1. Arc-resisting portions 7, 8 prepared from arc-resisting metal are respectively formed on the inner walls of the forward end portions of the arcing contact fingers 2 and that end portion of the stationary arcing contact 6 which faces the inner walls of the contact fingers 2. Where the conventional puffer type gas circuit breaker is shifted from a closed state indicated in 2 dots-dash lines to an open state indicated in solid lines, these portions of the arcing contact fingers 2 and stationary arcing contact 6 which the arc-resisting portions 7, 8 are disposed are prevented from being damaged by heat when exposed to arcs of large current produced between both arcing contacts 1, 6 when they are opened.
In recent years, the rated current capacity of a puffer type gas circuit breaker progressively increases as 40,000 to 50,000 A, 50,000 to 63,000 A, 63,000 to 80,000 A. As a result, an arc in the passage of an arc-extinguishing gas extremely increases in diameter and energy, resulting in the loss of the current-shutting-off property of the above-mentioned circuit breaker and the effective life of the various contacts. The customary practice to cope with such difficulties is to increase the force of pulling the puffer cylinder of the circuit breaker and/or increase the diameter of the puffer cylinder so as to compress the arc-extinguishing gas filled in the puffer cylinder to increase the pressure of the compressed gas for shutting off current more efficiently. However, this conventional process has the drawbacks that arcs 9 produced between the movable arcing contact 1 and the stationary arcing contact unavoidably expand, as shown in dotted lines in FIG. 1, beyond the arc-resisting portion 7 of the stationary contact 6 and extend beyond the stationary arc-resisting portion 8 to reach the portion of the stationary arcing contact 6 which is adjacent to the portion 8 and which is not protected by arc-resisting metal. Where, therefore, circuit breakage was repeated often by short circuit currents, the forward end faces of the arcing contact fingers 2 were damaged by the arcs as marked with the numeral 10 in a circle A of enlarged representation. As a result, the sharp edge 11 of the arc-resisting portion 7 protruded from the end face of the respective arcing contact fingers 2. Since the gradient of the potential in the proximity of the sharp edge 11 of the arcing contact fingers 2 became large, restriking of arcs sometimes appeared between the stationary arcing contact 6 and the sharp edge 11 of the arcing contact fingers 2 due to restriking voltage being produced immediately after the shutoff of current. Therefore, the known puffer type gas circuit breaker had the drawback that the current shutting-off characteristics were extremely lost.
Further disadvantages of the customary puffer type gas circuit breaker were that since the arc-resisting portion 8 was only formed on the furthest end of the stationary arcing contact 6, that section of the stationary arcing contact 6 which was adjacent to the arc-resisting portion 8 was damaged by the arcs and decreased in diameter as marked with the numeral 12 in a circle B. Where, therefore, the prior art circuit breaker was closed, the contacting pressure with which the movable arcing contact 1 and stationary arcing contact 6 were pressed against each other was reduced, resulting in unsatisfactory current passage. As a result, arc was produced between the movable arcing contact 1 and stationary arcing contact 6 and the section 12 of the stationary arcing contact 6 is damaged by the arcs with a decline in diameter, before these contacts 1, 6 reached the opening points, prominently shortening their effective life.